The invention relates to a circuit for suppressing a common mode component in a signal from a communication bus, comprising:
a first supply terminal and a second supply terminal for the connection of a supply voltage, PA1 a first bus terminal and a second bus terminal for the connection of respective wires of the communication bus, PA1 a voltage divider connected between the first bus terminal and the second bus terminal, PA1 a differential amplifier having inputs connected to taps of the voltage divider, PA1 a first current conducting element for coupling a center tap of the voltage divider to the first supply terminal, and PA1 a second current conducting element for coupling the center tap of the voltage divider to the second supply terminal.
Such a circuit is used in so-called Controller Area Network (CAN) bus systems, which are used inter alia in cars. Such a communication bus has transceivers (transmitter/receiver) connected to its first and its second bus terminal for the transmission and reception of information, which is transferred as a difference signal over the two wires of the communication bus. The two bus wires, which are commonly referred to as CANH and CANL, are often long and susceptible to high-frequency electromagnetic interference. Such interference has the same influence on both wires and consequently manifests itself as a common mode component in the difference signal.
In the customary circuit the voltage divider attenuates the signal voltages on the bus wires CANH and CANL in order to suppress the common mode component. By means of two current conducting elements, usually resistors, the center tap of the voltage divider is set to a fixed voltage, frequently half the supply voltage. The attenuated signal, to be processed by the differential amplifier, is available at the taps. As a result of the attenuation the useful difference signal is attenuated likewise. The choice of the attenuation factor is a compromise which depends on the common mode swing of the differential amplifier and the envisaged maximum common mode voltage. The detection of the useful difference signal becomes more inaccurate as the attenuation is higher.